UPDATE: Enemies who disappear when hit in multiplayer, don't REALLY disappear. They are still there, and still get damaged by the environment. However, they cannot attack you and you cannot attack them. I tested this by hitting an enemy, they "disappeared", and then I poured lava right on top of me and they got hit by it, which caused them to re-appear. So hitting the disappeared enemies with lava causes them to re-appear, which then allows them to get hit normally again.
Does that help pinpoint the issue so it can be fixed? It's really hard to play multiplayer since many modded enemies still disappear...
Also, when summoning any Tremor boss in multiplayer, this happens:
Any idea on how to fix this? Or do you need to fix it on your end? Enemies disappear as soon as they are hit in multiplayer, too.